Command-curable composition

ABSTRACT

A light-curable dental cement composition contains (a) the reaction product of vinyl phosphonyl chloride and pentane 1,5 diol, (b) a photopolymerisation initiator, (c) ion-leachable aluminosilicate glass powder, (d) poly(vinyl phosphonic acid), (e) bisGMA as a photopolymerisable dimethacrylate resin, and water, inert filler and silanising agent. Upon mixing, glass is added last, the composition is put in place and, when ready, it is light-irradiated for 30 or 40 seconds to polymerise (e). The composition is immediately firm and the phosphonate groups adhere to tooth material while further slow curing by acid-base reaction continues.

This invention relates to compositions curable on command, e.g. by theaction of incident energy e.g. light (ultraviolet or visible) orultrasound. Such compositions may contain inert or reactive fillers, inwhich case they can be classed as cements, and may be particularlyuseful in surgical, especially dental, applications.

As dental cements, glass ionomers as described in for example GB Patents1422337 and 1484454 have attained wide popularity for their compressivestrength, their inherent adhesion to tooth material, their relativelyfast setting time and their anti-caries action. However, a drawback inclinical practice is that, once mixed, the glass ionomer cementcomposition stays workable for a strictly limited time only, and setsrapidly.

It has been proposed in EP-A-323120 to formulate a glass ionomer withmodifying agents to delay its setting, with photocurable characteristicstoo, so that a photocure can be effected by the dentist when he isready, with the acid-base glass ionomer reaction proceeding in parallel.However, those formulations having acceptable adhesiveness remainworkable for only about five minutes, which could still be too short todeal with unexpected clinical eventualities.

Nonetheless, light-cured glass-ionomer cements have now become widelyavailable for use as lining materials. The photocurable characteristicsare supplied by adding, to the glass and polyacid required for theacid-base reaction, polymerisable monomers such ashydroxyethylmethacrylate (HEMA) which participate in thephotopolymerisation reaction.

These light-cured glass-ionomers have proved to be very sensitive to thepresence of moisture. This is because the set cement has a structuresimilar to that of a hydrogel. The structure is lightly crosslinked andcontains hydrophilic groups, the light-cured methacrylates. Hence thecement has an affinity for water, which, once absorbed, acts as aplasticiser within the materials and reduces the strength of the cement.The water also tends to swell the cement.

WO-A-88/05651 discloses a light-curable glass ionomer lining materialdetectable by X-rays, comprising a fluoroaluminosilicate glass, bariumglass, benzoyl peroxide and a resinous binder particularly bis-GMA orurethane dimethacrylate. This WO lists desirable criteria which thisliner should meet as including translucency, formulation as a two-pastesystem, long shelf life, low solubility in oral fluids, strong adhesionto tooth, suitable working and setting times, and adequate strength.

EP-A-0 395 427 discloses a phosphonate ester which will cure on exposureto light in the presence of an initiator, this being mixed withpoly(vinyl phosphonic acid), glass powder and water, to form alight-curable glass ionomer as already mentioned, which can remainworkable for some 30 minutes in normal indoor light.

It would be desirable to have a command-curable composition which,despite comprising a number of dissimilar chemical compounds, waspresented in a stable non-separating common solution and which, whenset, did not tend to swell upon the absorption of water.

According to the present invention, a command-curable compositioncomprises (a) a partly esterified phosphonic acid being equivalent tothe reaction product between a precursor of a polyphosphonic acid (e.g.poly(vinyl phosphonic acid)) and an esterifying group such as apolyhydric alcohol in the mole ratio (0.2-5.0) phosphonic acid groups: 1esterifying group, (b) an initiator suitably a light-activated initiatorsystem or a chemical initiator system, (c) cation-leachable glasspowder, (d) poly(vinyl phosphonic acid) and is characterised by (e) abulky molecule compatible with component (a) and having at least twovinyl groups through which it may cross-link/polymerise to yield acompletely hydrophobic polymer. The composition preferably furthercomprises water. The said mole ratio is preferably (0.5-1.5):1 such as(0.8-1.2):1. The precursor may be vinyl phosphonic dichloride. The OHgroups in the alcohol are preferably interconnected via from two totwenty such as two to six organic backbone (usually carbon atoms), withoxygen atoms optionally interposed at least (preferably) every fifthcarbon atom. The bulky molecule may be a dimethacrylate such as adimethacrylate formed from the reaction between bisphenol A and glycidylmethacrylate.

The cation-leachable (e.g. aluminosilicate) glass powder may be partlyor wholly replaced by amphoteric or basic metal oxide (e.g. MgO, whichmay be de-activated at at least 900° C.). By "poly(vinyl phosphonicacid)" we include any multivalent-cation-catalysed cross-linkablepolymeric acid containing on average one phosphonic acid group per oneto three backbone carbon atoms. A minor proportion of poly(carboxylicacid) such as poly(acrylic acid) may also be present. This compositionwhen mixed together will cure slowly by ionomer (acid-base) reactionbetween the phosphonic acid groups and the glass/oxide, and rapidly ondemand by polymerising (cross-linking) reaction of the bulky moleculewhen initiated by (b).

The invention extends to a pack comprising the components of saidcomposition (optionally excluding water) so presented as not to reactprematurely, for example in the form of two separated pastes which whenmixed form a curable composition as set forth above; the first paste maybe the acid(s) plus water and the light-activated orultrasound-activated initiator system, and the second paste may be theglass powder suspended in the dimethacrylate. If the two pastes havebeen formulated to appropriate concentrations, one could in use squeezeout equal lengths of paste from two tubes, or scoop out equal numbers ofspoonfuls from two tubs, as an easy way to ensure that the mixture is ofthe correct composition.

The glass powder preferably consists of particles substantially all ofwhich are smaller than 100 microns, preferably smaller than 60 microns.The Si:Al atomic ratio range of (0.6-2):1 yields an opaque product,which may be acceptable in appropriate cases, but (0.2-0.6):1 can alsobe used.

Also present may be fillers, preferably inert e.g. quartz, and/or asilanising agent to improve the incorporation of the filler into thecement. The bulky molecule can yield a completely hydrophobicpolymer--thus stable against oral fluids--which can however successfullywet glass, improving the coherence of the set cement. The cement canalso be compatible/adhesive to tooth material through phosphonate groupsin the acid and in the esterified acid, the later being compatible withthe bulky molecule yet itself being polar and thus hydophilic. Theinvention will now be described by way of example.

The components used in the following examples of light-cured, poly(vinylphosphonic acid) glass ionomer cements are:

1. Bis GMA

Bis GMA is the main photopolymerisable component in the cement. It is adimethacrylate resin formed from the reaction between bisphenol A andglycidyl methacrylate.

2. PR

PR is the product of reaction between vinyl phosphonyl dichloride andpentane 1,5 diol, as described in EP-A-395427. PR is anotherphotopolymerisable component of the cement, although its degree ofphotopolymerisation is low for electronic reasons. Within the cement thePR should act as an adhesion promoter, also promoting compatibility andmiscibility between the other components of the cement.

3. Initiator

This, the photopolymerisation initiator, is made up of camphorquinone(CQ), sodium toluenesulphinate (NaTS) and ethyl p-dimethylaminobenzoate(EDMAB). The components of the initiator are mixed in the ratioCQ:NaTS:EDMAB=0.3:2.0:0.75.

4. Water

Water is required in the cement formulation as the medium for iontransport in the acid-base reaction.

5. PVPA

PVPA is the polyacid, poly(vinyl phosphonic acid), used in the cementformulation. It is required for the conventional acid-base reaction inthe cement formation.

6. Glass

The glass is an ion-leachable aluminosilicate. In the formation of thecement it acts as the base for the acid-base reaction. The glass isprepared by mixing together 437 parts by weight silica, 230 parts byweight alumina, 129 parts by weight calcium fluoride, 175 parts byweight cryolite and 29 parts by weight aluminium phosphate and heatingto 1300° C. for 75 minutes. The melt is cooled rapidly by pouring intowater. The resulting glass is ground and sieved, and the fraction ofparticle size less than 45 microns used in the composition.

For each example, the same mixing regime was followed:

All the components of the cement were weighed out ready. First theBisGMA and PR were combined on an inert surface. Then the Initiator wasmixed in, followed by the Water and then the PVPA. When an homogeneouspaste had been formed, the Glass was added as the final component. Forthe purposes of the determination of working time, time t=0 was taken aswhen the Glass was added.

Layered compressive strength specimens were prepared in 4×6 mm moulds;each layer was packed and then irradiated to cure it using dental bluelight (35 W halogen lamp) as close as possible to the specimen butensuring that the specimen did not touch the light. The cements werestored in the moulds at 37° C. for one hour and then stored in water at37° C. for a further 23 hours. The load at failure in compression wasmeasured using an Instron 1185 with crosshead speed 1 mm/min, from thisvalue the measured compressive strength can be ascertained.

The duration of the irradiation to cure, the amount used of eachcomponent in parts by weight, and the resulting compressive strength inmegaPascals (with the standard deviation), are shown for each Example inthe following table.

    __________________________________________________________________________    Ex No                                                                             cure                                                                              BisGMA                                                                             PR  Initr                                                                            Water                                                                             PVPA Glass                                                                             CS  (sd)                                     __________________________________________________________________________     1  30s 0.13 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.20                                                                              21.70                                                                             (2.1)                                     1a.sup.†                                                                  30s 0.13 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.20                                                                              15.76                                                                             (1.6)                                     2  30s 0.13 0.07                                                                              0.09                                                                             0.05                                                                              0.10 0.30                                                                              18.30                                                                             (4.7)                                     2a.sup.†                                                                  30s 0.13 0.07                                                                              0.09                                                                             0.05                                                                              0.10 0.30                                                                              15.94                                                                             (2.6)                                     3  30s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.20                                                                              27.19                                                                             (2.3)                                     4  30s 0.16 0.07                                                                              0.08                                                                             0.08                                                                              0.10 0.20                                                                              17.04                                                                             (2.0)                                     5  30s 0.13 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.30                                                                              19.59                                                                             (3.1)                                     6  30s 0.10 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.20                                                                              9.78                                                                              (1.8)                                     7  40s 0.13 0.07                                                                              0.06                                                                             0.05                                                                              0.10 0.20                                                                              17.97                                                                             (2.0)                                     8  40s 0.13 0.07                                                                              0.06                                                                             0.05                                                                              0.10 0.30                                                                              33.58                                                                             (5.3)                                     9  30s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.30                                                                              23.10                                                                             (4.9)                                    10  30s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.40                                                                              30.90                                                                             (3.3)                                    10a 40s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.40                                                                              34.53                                                                             (4.9)                                    11  30s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.45                                                                              29.00.sup.                                                          (7.5)                                    11a 30s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.45                                                                              30.60                                                                             (2.6)                                    11b 40s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.45                                                                              31.45                                                                             (3.2)                                    12  30s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.50                                                                              30.98                                                                             (6.1)                                    12a 40s 0.16 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.50                                                                              32.92                                                                             (3.9)                                    13  30s 0.13 0.07                                                                              0.08                                                                             0.075                                                                             0.15 0.45                                                                              23.28                                                                             (3.3)                                    14  30s 0.13 0.07                                                                              0.08                                                                             0.075                                                                             0.15 0.55                                                                              26.44                                                                             (1.6)                                    15  30s 0.20 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.55                                                                              38.67*                                                                            (4.0)                                    15a 40s 0.20 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.55                                                                              41.74∇                                                                   (4.6)                                    15b.sup.§                                                                    30s 0.20 0.07                                                                              0.08                                                                             0.05                                                                              0.10 0.55                                                                              34.87                                                                             (5.0)                                    __________________________________________________________________________     Notes                                                                         .sup.† stored in air                                                   .sup.§ storage: in mould 37° C. 1 hr; in moist atmosphere         37° C. for further 23 hrs; in water 37° C. for next 24 hrs      high value recorded 43.34 MPa                                   *high value recorded 45.83 MPa                                                ∇high value recorded 49.67 MPa                                  

The wide variation in these compressive strength values may be due tovarious extraneous factors, such as the difficulty in consistentlypacking layers in the mould (specimens yielding lower strengths whenthere were obvious flaws in the sides of the specimen) and the timeelapsed between mixing and placing, and irradiating, during which time aglass ionomer matrix may have formed to a sufficient extent to impedeformation of the polymerisation matrix. If this is the case then the twomatrix forming reactions are in competition. Consequently it is possiblethat difference layers within the same cement may have differentproportions of acid-base and polymerisation matrices and hence differentstrengths.

The presence of PVPA (preferably at least 11/4, e.g. at least twice, themass of the water) and of the PR should improve the adhesion of thecement to tooth material. BisGMA undergoes the main light-inducedpolymerisation and is used in this formulation because it has a goodpercentage conversion on illumination and also because as a result ofits bulky size the hydrophilicity and polymerisation shrinkage aresmall.

Effect of BisGMA

Comparing Examples 1, 3 and 6 shows that increasing the levels of BisGMAincreased the strength of the cement. It was preferably at least 11/2times, more preferably at least twice the mass of the PVPA.

Effect of Level of Initiator

It is important that the level of initiator be as low as possible in thecement, otherwise the components of the initiator can leach our from theset cement and cause a cytotoxic response. A reduction in the level ofinitiator may need to be accompanied by an increase in irradiation time,which is allowable provided that the longer irradiation time is stillacceptable clinically. Examples 1, 2 and 7 suggest an optimum around 13%by mass of initiator, which is high.

Effect of Cure Time

Increasing the cure time of the material did not appear to cause astatistically significant increase in strength for Examples 10/10a,11/11b, 12/12a nor 15/15a. This would imply that for these materials the30 seconds' cure time is sufficient.

Effect of Level of Glass

The glass acts as both the filler in the photopolymerisation reactionand the base in the acid-base reaction. While increasing the level ofglass in the system should improve the strength of the cement, Examples9, 10, 11 and 12 show a trend which is not significant statistically.Glass was preferably more than four, more preferably more than five,times the mass of the PVPA, especially advantageously with high bisGMA.

BisGMA, being a dimethacrylate resin formed from the reaction betweenbisphenol A and glycidyl methacrylate, has the structure ##STR1##

Hydroxyethylmethacrylate (HEMA), a significantly smaller molecule, hasthe structure ##STR2##

Both are polymerised through the vinyl (CH₂ =C<) group. Dimethacrylatessuch as bisGMA give crosslinked polymers, while HEMA forms linearpolymers which may however show branching.

We claim:
 1. A command-curable composition comprising:(a) a partlyesterified phosphonic acid being equivalent to the reaction productbetween a precursor of a poly(vinyl phosphonic acid) and a polyhydricalcohol, in the mole ratio (0.2-5.0) phosphonic acid groups: 1esterifying group; (b) an initiator; (c) cation-leachable glass powderand/or amphoteric or basis metal oxide; (d) poly(vinyl phosphonic acid);and (e) a bulky molecule compatible with component (a) and having atleast two vinyl groups through which it may cross-link/polymerize toyield a completely hydrophobic polymer, said bulky molecule being adimethacrylate; said partly esterified phosphonic acid (a) being presentin an amount to promote adhesion and to promote compatibility andmiscibility of the other components of the composition; the weight ratioof component (d) to component (c) being from 1:2 to 1:5.5; and theweight ratio of component (d) to component (a)+component (e) being from1:1.3 to 1:2.7.
 2. A command-curable composition according to claim 1,further comprising water.
 3. A command-curable composition according toclaim 1, wherein the OH groups in the alcohol are interconnected viafrom two to twenty organic backbone atoms.
 4. A command-curablecomposition according to claim 3, wherein the OH groups in the alcoholare interconnected via carbon atoms with oxygen atoms interposed atleast every fifth carbon atom.
 5. A command-curable compositionaccording to claim 3, wherein the said mole ratio is (0.5-1.5):1.
 6. Acommand-curable composition according to claim 5, wherein the said moleratio is (0.8-1.2):1.
 7. A command-curable composition according toclaim 1, wherein the said precursor is vinyl phosphonic dichloride.
 8. Acommand-curable composition according to claim 1, wherein component (c)is aluminosilicate glass powder.
 9. A command-curable compositionaccording to claim 8, wherein the glass powder (c) has an Si:Al atomicratio of (0.2-0.6):1 or (0.6-2):1.
 10. A command-curable compositionaccording to claim 1, wherein the glass powder consists of particlessubstantially all of which are smaller than 100 microns.
 11. Acommand-curable composition according to claim 1, wherein component (c)is MgO deactivated at at least 900° C.
 12. A command-curable compositionaccording to claim 1, further comprising a filler.
 13. A command-curablecomposition according to claim 1, further comprising a silanizing agent.14. A pack comprising all the components of a composition according toclaim 1, so presented as not to react prematurely.